1) Create a mission and put yourself on one side of the map in the editor and save it.
2) Then save the mission also as a SP/MP pbo file.
3) Go to host a game and you will see both versions available. Click on the full file version (blue text) and hit edit.
4) This will launch the editor. Move yourself to the other side of the map and hit 'Preview'
5) The mission will launch, but you will start at the original location and not the location you just moved the unit in the editor too (because it's loading the original .pbo version of the mission and not the file version you just edited.